Boiler drum structure for rapid temperature changes

ABSTRACT

The steam drum of a high pressure steam generating unit, and particularly one subject to frequent thermal cycling, is divided into compartments. Each compartment is, to a large extent, fluidly isolated from the others. Furnace wall steam-water mixture is admitted to an intermediate compartment which is spaced from the inner surface of the drum wall by an annular outer compartment extending about substantially the entire inner circumference of the drum. The majority of the steam-water mixture is conducted, via the intermediate compartment, through steam separators and into an inner compartment. A small part of the mixture may be allowed to pass from the intermediate compartment into the outer compartment and then into the downcomer to avoid the formation of a water level in the outer compartment.

lJit

Tuppeny, Jr.

6S atent I [75] Inventor: William Henry Tuppeny, Jr.,

Rockville, Conn.

[73] Assignee: Combustion Engineering, Inc.,

Windsor, Conn.

221 Filed: Aug. 25, 1971 21 Appl.No.: 174,822

. 1 June 19, 1973 Primary Examiner-Kenneth W. Sprague Attorney-Eldon H. Luther and Stephen A. Schneeberger [57] ABSTRACT The steam drum of a high pressure steam generating unit, and particularly one subject to frequent thermal cycling, is divided into compartments. Each compartment is, to a large extent, fluidly isolated from the others. Furnace wall steam-water mixture is admitted to an intermediate compartment which is spaced from the [52] U s C] 122/235 Z 122/491 inner surface of the drum wall by an annular outer [51] lm i 37/26 compartment extending about substantially the entire [58] a 122235 Z 488 49] inner circumference of the drum. The majority of the steam-water mixture is conducted, via the intermediate [56] References Cited compartment, through steam separators and into an inner compartment. A small part of the mixture may be UNITED STATES PATENTS allowed to pass from the intermediate compartment 2,675,888 4/1954 Bhzard et a] 122 491 into the outer compartment and then into the down 2,724370 11/1955 K comer to avoid the formation of a water level in the 2,743,709 5/1956 rmacost outer compartment 6 Claims, 4 Drawing Figures I J 24 JJ-IIJ-BMJ 2::12222- 223 Lg? M55: 26

' Patented June 19, 1973 3,739,752

3 Sheets-Sheet 1 FIG. I

Patented June 19, 1973 3,739,752 I 3 Sheets-Sheet 3 gmmgmmgfimgnmg W U Q mmmmmmmg FIG. 3

FIG. 4

BOILER DRUM STRUCTURE FOR RAPID TEMPERATURE QI'IANGES BACKGROUND OF THE INVENTION The invention relates to improvements in boiler drum design and is more particularly concerned with eliminating or greatly reducing thermal stresses in the wall of the steam and water drum of high pressure steam generating units when these units are put into or taken out of operation.

Many modern vapor generating units operate at high fluid pressures of the order of 2,000 3,000 psi and it is evident that these units, in order to withstand such pressures over a long life, must have their pressure components of a considerable wall thickness. This is particularly true of the steam and water drum of the pertinent units. Some of these drums have a length of the order of 50 90 feet and a diameter of the order of 4 7 feet. Under such conditions, the wall thickness of the steel drum wall must be great. It is, in many cases, of the order of 4 10 inches.

In starting up a high pressure boiler primary consideration must be given to maintaining a uniform temperature throughout the drum so as to avoid excessive thermal stresses in the drum wall. In the past this has been accomplished by slowly raising the pressure of the unit and thereby the temperature of the drum uniformly. This procedure obviously increases considerably the time required to start up a steam generating unit operating under high pressure. If this starting up and shutting down procedure must be repeated frequently such as every day or every other day, it becomes extremely important from the standpoint of power plant operating efficiency to preform this operation as rapidly as possible.

One boiler drum arrangement which made significant strides in reducing the time required for starting up and shutting down a boiler is that disclosed in Armacost US. Pat. No. 2,743,709 of May I, 1956. The boiler drum design of the Armacost patent permitted a fluid temperature change rate which greatly exceeded the previously accepted maximum of 100 per hour and in fact permitted temperature change rates in excess of 350 per hour for a limited number of occurrences.

However, with the increasing emphasis on the use of high pressure boilers for daily peaking operation there has been an emphasis on the economical advantages to be derived from still further decreasing the time required for start up and shut down of a boiler, while insuring system integrity over a life of 5,000 10,000 cyclingoperations. It can be well appreciated by one familiar with power plant economics that considerable savings in operating costs such as fuel and labor can be realized if this start up period (and shut down period) can be shortened by even a small amount.

Attempts to reduce the start up time of high pressure boilers have resulted in severe thermal stressing of the boiler drum and because of the relatively frequent occurrence of such start ups has, in some instances, resulted in the failure of the drum. Additionally, it should be noted that while the drum design of the earlier mentioned Armacost patent does result in a reduction of the thermal gradient experienced about the circumference of the drum, it does not significantly reduce the thermal gradient in a radial direction through the wall of the drum and thus permits unwanted stressing of the drum wall. High pressure steam generating systems operating at a steam pressure of nearly 3,000 lbs. per square inch have served to further aggravate this problem by requiring steam drums having a wall thickness which may approach 10 inches and by requiring a greater amplitude to the thermal cycle during cycling of the system.

SUMMARY OF THE INVENTION It is an object of the invention to provide a stem and water drum for a high pressure steam generating unit capable of repeatedly and frequently receiving furnace wall tube fluid flow at temperatures substantially different than those of the drum wall.

The present invention provides means for delivering steam and water from the furnace wall tubes of a high pressure steam generating unit to the interior of a drum in a manner which minimizes thermal stressing of the drum wall. Baffles or partitions spaced from both the inner wall of the drum and from one another extend longitudinally of the drum substantially the entire length thereof to divide the interior of the drum into outer, intermediate, and inner compartments. The partitions are spaced from and sealed in fluid tight relation to one another to form the intermediate compartment. The steam-water flow from the furnace wall tubes is conducted through the drum wall by means of thermal sleeves and directly into the intermediate compartment spaced from the inner wall of the drum. The intermediate compartment conducts all or most of the steamwater flow through steam separators and into the inner compartment. The outer compartment serves to thermally isolate the drum wall from the water-steam flow in the intermediate compartment and the steam in the inner compartment.

The several baffles or partitions are preferably formed of thin section material and may be of a material having a coefficient of expansion comparable to that of the drum but having a thermal conductivity substantially less than that of the drum. Further, a limited portion of the steam-water flow in the intermeidate compartment may be permitted to exit into the outer compartment through holes in the partition between the intermediate and outer compartments to minimize the pressure difference across the outer partition and to allow a limited flow of fluid in the outer compartment. The inner and outer compartments communicate directly with one another only near the bottom of the drum, and the inner compartment is filled with water to a level above said communication between said inner and outer compartments. The communication between inner and outer compartments may take the form of an aspirator which uses the velocity of water flowing from the inner compartment out of the drum and into a downcomer for creating a suction at the lower end of the outer compartment whereby the limited amount of fluid entering the outer compartment from the intermediate compartment is continuously removed therefrom to avoid the formation of a liquid level therein.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view indicating a type of high pressure vapor generating unit to which the invention applies;

FIG. 2 is a transverse section through the steam and water drum enlarged to show how water and steam flow controlling devices may satisfactorily be installed therein to accomplish the objects of the invention;

FIG. 3 is a reduced scale schematic horizontal longitudinal section through the steam and water drum (taken on line 3-3 of FIG. 2) and showing how the herein disclosed intermediate compartment is supported and positioned to effect the requisite separation of the steam-water flow from the drum wall; and

FIG. 4 is a partial vertical longitudinal section of the steam and water drum taken on the line 4-4 in FIG. 2 and showing the herein disclosed arrangement of compartments and aspirators.

DESCRIPTION OF THE PREFERRED EMBODIMENT The inventive apparatus herein disclosed may be exemplified in conjunction with a high pressure radiant type boiler operating with forced circulation and having a single water and steam drum, and will be described with reference thereto. It will be evident that the drum configuration of the invention may also be used with waste heat boilers and other types and that the steam and water may be other vapors and liquids.

A typical embodiment of such a boiler has an output of 4 X 10 lbs. of steam per hour and may experience a temperature as great as 680F and a pressure as great as 2,900 lbs. per square inch under conditions of maximum load. This boiler is typically equipped with a steam and water drum having an internal diameter which is approximately 5% feet and is 50 to 90 feet long. Drum is formed of steel and has a wall thickness of approximately 8 inches. Apparatus internal to and associated with drum 10 incorporates the new features of this invention and will be more fully described hereinafter.

Referring to FIG. 1 which diagrammatically depicts a vertical sectional view of a type of high pressure vapor generating unit to which the invention is applied, there is shown a vertically elongated combustion chamber portion 12 of furnace 13 which is defined by upright vapor generating wall tubes some of which are indicated at 14 and leading from bottom header 16 to the top header 18. A steam and water separating drum 10 is physically removed from furnace chamber 13 and supported by suitable hangers 20. Wall tubes 14 comprise the major portion of a water vaporizing circuit having both its inlet and outlet in drum 10. Makeup or feed water is fed under pressure by means of pump 22 to the economizer 24 wherein it is heated and flows via conduit 26 to a lower portion of steam and water drum 10. The water flowing to drum 10 from economizer 24 is distributed along the length thereof by feed pipe 28 (see FIGS. 2, 3, and 4) located in drum l0, partly filling portions of the drum to the water level indicated at 30.

From the drum 10 the water enters downcomer conduit 32 leading to forced circulation pump 34 which forces the water through headers 16 and through wall tubes 14 whereat heat received from the combustion chamber 12 vaporizes a portion of the water flow. The resulting steam-water flow in wall tubes 14 is collected in headers 18 and is conveyed through conduits 36 to drum 10 and is introduced thereinto by means of the invention to be subsequently described. Within drum 10 a separation of the steam-water flow occurs with the water occupying the lower portion of the drum and the steam occupying the upper portion. The steam leaves drum 10 through conduit 38 and is delivered to header 40 whereupon it is passed through low temperature stage 42 of a two stage superheater and into header 44 and then is conveyed to the high temperature stage 46 of said superheater via header 48 from which it passes to superheater outlet header 50 whereupon it enters the plants main stream line and is available for driving a turbine.

In recent years with the greatly increased usage of electrical power, the load requirements on a typical electrical power plant have been seen to vary by large amounts on an almost daily basis. Typically the load requirements on a public utility system supplying power to a metropolitan area may differ by more than a factor of 3 from low load to high load. This load variation often occurs with a daily cycle and is influenced by such factors as weather conditions and days of the week. In an effort to adequately and economically accommodate such widely varying loads, utilities often operate some steam generators under base loaded conditions and others under peaking or cycling conditions. The subject invention is particularly applicable to the steam drums of the latter type of system wherein rapid startup and shut down of a steam generating system may occur on a daily basis, though it also finds applicability during the less frequent start ups and shut downs of base loaded systems. Because of the economics involved, steam generating systems which are cycled on a daily basis preferably come from a cold or hot stand condition to full load conditions in less than an hour. As an example, a steam generating system may be bottled up or placed in a hot standby condition wherein the unit fluid pressure is about 500 psi (corresponding to a saturation temperature of 465) and then during therapidly increasing load demand in the morning hours be required to attain a steam pressure in excess of 2,500 psi and saturation temperature in excess of 650F in less than 30 minutes. From a cold start up, the starting temperatures and pressures are obviously lower and temperature rises of from 400 600 per hour may occur. From these statistics, it becomes evident that the thick walls of a steam drum may be subjected to very significant thermal gradients caused by the furnace wall tube flow entering the drum. As system pressures and accordingly steam drum wall thicknesses have increased, the problem of large thermal gradients and severe thermal stressing of the drum has become more acute. While such stressing, if infrequent, might be tolerable, such stressing occurring 200 '300 times per year may create a dangerous condition of fatigue in the drum wall early in its life. The steel wall of drum 10 having an 8 inch thickness should be subjected only to a relatively low temperature gradient, however, because steel is a relatively poor thermal conductor and the wall under cold conditions may have a temperature of only 200 300F and is quite thick, contact of the inner drum wall surface with large quantities of steam and water at temperatures 200 300 greater than the outer wall surface temperature will greatly exceed this temperature gradient.

According to the invention, means are provided for preventing or moderating the direct contact of the furn'ace wall tube fluid with the wall of drum 10. The water-steam mixture delivered to drum 10 would, in prior art steam drum designs, come into contact with the inner wall surface of the drum, however, this is substantially prevented by the drum structure of the invention.

According to the invention and as seen in FIG. 2, the interior of drum is divided into three distinct zones or compartments formed by partition means such as outer partition 54 and inner partition 56. Outer partition 54 extends longitudinally of drum 10 and is comprised of a plurality of baffle plates 58 joined to one another in a fluid tight manner as by welds 60. Similarly the inner partition 56 also extends longitudinally of drum l0 and is comprised of a plurality of baffle plates 58 joined to one another in a fluid tight manner as by welds 60. Both outer partition 54 and inner partition 56 generally conform to the curvature of the inner wall surface of drum 10 with outer partition 54 being spaced therefrom about one-half inch and inner partition 56 being spaced therefrom about 2% inches or 2 inches from outer partition 54. Both outer partition 54 and inner partition 56 are supported at their upper ends by being joined, as by welding, to the steam collection channel 62. Steam collection channel 62 contains steam dryers 64 and is affixed to steam pipes 66 which are part of steam conduit 38. Outer partition 54 and inner partition 56 extend downwardly from the upper area of the drum on both sides thereof to near the bottom of the drum whereat they are supported as by support elements 68. Support elements 68 may be positioned at various locations within drum 10 to provide the needed spacing and support for members therein and they have geometries which permit fluid passage thereby, as for instance, a split support ring. This arrangement of inner and outer partitions generally divides the interior of drum 10 into an outer compartment 70, an intermediate compartment 72, and an inner compartment 74. Both outer and intermediate compartments, 70 and 72 respectively, are substantially annular in shape and extend longitudinally of drum l0 and inner compartment 74 is substantially cylindrical in geometry. It is to be realized that the cross sectional geometries of the several compartments may differ somewhat from purely annular or cylindrical, so long as the resulting thermal gradients through and around the wall of drum 10 are within acceptable limits during the various modes of operation.

The steam collection channel 62 extends longitudinally of drum 10- and extends across the upper end of the intermediate compartment 72 to form a closure therefor and outer partition 54 and inner partition 56 are joined thereto in a fluid tight manner as be welding. Closure means, such as closure plates 78 discussed hereinafter, extend between partitions 54 and 56 and are affixed thereto in a fluid sealing manner to substantially fluidly isolate intermediate compartment 72 from outer and inner compartments 70 and 74.

Fluid communication between inner compartment 74 and outer compartment 70 and additionally between inner compartment 74 and downcomer conduit 32 is provided by means of a fluid conduit such as cylinder 76 extending between inner compartment 74 and outer compartment 70 and in registry with a nozzle 77 of downcomer conduit 32. Generally steam drum 10 will have a plurality of downcomer nozzles 77 at the ends of conduits 32 and communicating with the drum interior at the lower portion thereof as seen in FIGS. 2, 3 and 4 and a communication cylinder 76 is positioned axially in registry with each of the downcomer nozzles 77. Communication cylinders 76 are open at each end with the upper end in fluid communication with inner compartment 74 and with the lower end extending downwardly a substantial distance into a downcomer nozzle 77. The outside diameter of cylinder 76 is less than the inside diameter of downcomer conduit 32 and is spaced from the inner surface thereof to provide an annular space 79 between the inner surface of downcomer nozzle 77 and the outer surface of cylinder 76. Cylinders 76 extend through circular openings in the lower portions of partitions 54 and 56 and are welded thereto to create a fluid tight seal, thereby maintaining the integrity of the intermediate compartment. The function of communication cylinder 76 will be described in greater detail hereinafter.

Outer compartment preferably extends adjacent substantially the entire inner circumference of drum 10. Intermediate compartment 72 also preferably extends adjacent substantially the entire inner circumference of outer partition 54 with the exception of that space near the upper most part of drum 10 occupied by steam collection channel 62. As best seen in FIGS. 3 and 4, either outer partition 54 of inner partition 56, or both, extend longitudinally the full entirety of drum 10 and are secured at each end to the inner surface of the drum end wall in a fluid tight seal, as by welding. This arrangement serves to isolate outer compartment 70 from inner compartment 74 as required by the invention with the only direct communication therebetween occurring through communication cylinders 76. Closure plates 78, as seen in FIGS. 3 and 4, extend between and are joined to outer partition 54 and inner partition 56 in a fluid tight seal, as by welding, to substantially isolate intermediate compartment 72 from outer and inner compartments 70 and 74 respectively. Closure plates 78 are positioned near the longitudinal end extremes of partitions 54 and 56 but are spaced from the inner surface of drum 10 by an amount comparable to the spacing between the drum wall and outer partition 54 for reasons which will hereinafter become evident.

As earlier mentioned feedwater is introduced to drum 10 by means of feed pipe 28 which is located within the inner compartment 74 of the drum and extends longitudinally thereof. The feedwater initially establishes and subsequently continues to maintain a preestablished water level 30 within the inner compartment 74. This water is supplied to the vaporizing circuit earlier described through downcomer conduit 32 which communicates with the inner compartment through a plurality of nozzles 77 extending through the bottom of drum 10. The water is partially vaporized in the wall tubes 14 of furnace l3 and is delivered to steam drum 10 through conduits 36. The fluid flowing through furnace wall tubes 14 and subsequently through conduit 36 enters drum 10 by means of thermal sleeves 80. Multiple thermal sleeves 80 extend longitudinally of drum 10 in one or several rows on each side of the upper portion thereof. Thermal sleeves 80 are comprised of a thick walled outer cylinder which is welded at 82 to the outer walled surface of drum 10 and a thin walled inner sleeve which extends through the thick wall of drum 10 to an opening in outer partition 54 in registry therewith. The thin walled inner sleeve is joined to outer partition 54 by means of a fluid tight seal 84 thus placing steam-water conduit 36 in direct or immediate communication with only the intermediate compartment 72. The steam-water mixture entering intermediate compartment 72 passes through the compartment as indicated by the arrows and is discharged to the inner compartment 74 by way of steam separating units 86 which communicate with intermediate compartment 72 through openings in the inner partition 56 in registry therewith. The steam and water are separated from one another in separators 86 in any suitable manner as by centrifugal action. The separated water falls to the bottom of inner compartment 74 as a portion of the water which establishes level 30 and the steam passes through dryers 64 and pipes 66 to conduit 38. The lower extremity of the intermediate compartment 72 and additionally the lower portion of steam separating units 86 preferably extend below the level 30 of water maintained in inner compartment 74.

The pressure drop in the steam-water flow entering drum 10 between its entry to the drum at thermal sleeve 80 and its discharge to the inner compartment 74 at steam separators 86 will normally be less than 10 psi. This permits the baffle plates 58 of inner partition 56 to be of relatively thin material, for instance Va inch steel. Were some direct fluid communication not present between outer compartment 70 and inner compartment 74 and/or intermeidate compartment 72, the baffle plates 58 which make up outer partition 54 would need to be of a substantial thickness to accommodate large pressure differences thereacross. However, the communication provided between outer compartment 70 and inner compartment 74 by means of cylinder 76 prevents creation of large pressure differences therebetween, thus allowing baffle plates 58 of outer partition 54 to be similar in thickness to those of inner partition 56.

However, because fluid communication does exist between outer compartment 70 and inner compartment 74, there is created the opportunity for a liquid level to become established in outer compartment 70. As an object of the invention it is desired to maintain the temperatures over the entire inner surface of the wall of drum 10 as uniform as possible but the existence of a water level in outer compartment 70 creates a liquid medium near the bottom of drum 10 having a different thermal conductivity than that of the gaseous medium in the remaining upper portion of outer compartment 70. The two differently conductive media between intermediate compartment 72 and the drum wall at different zones about its inner circumference will permit thermal stresses to be created in the drum wall at the gas-liquid interface. Further, the water level which may be established in outer compartment 70 is also subject to variation with changes in pressure, thereby creating additional undesirable effects.

In order to avoid the formation of a water level in compartment 70 communication is provided between intermediate compartment 72 and outer compartment 70 comprising a plurality of holes 90 through outer partition 54 in rows extending longitudinally of drum 10 near the top of said outer partition on both sides thereof. Holes 90 are each of small area and serve both a pressure equalizing function and additionally as flow orifices which permit a small determinable quantity (about 1 percent) of the steam-water flow in intermediate compartment 72 to be introduced into outer compartment 70 in the upper region thereof. Because the quantity of fluid introduced to outer compartment 70 in this manner is quite limited, any adverse thermal effects on the wall of drum 10 are also minimal, however, this limited flow does serve to establish a fluid flow path downward about the inner circumference of the wall of drum 10 as indicated by arrows 92. This flow exists because the fluid pressures in the area of cylinder 76 and downcomer nozzle'77 will be somewhat less than those existing in the water-steam mixture at holes 90. This flow in outer compartment is further assisted by the geometry and positioning of cylinder 76. Cylinder 76, extending downwardly into downcomer conduit 32 and having annular space 79 disposed concentrically thereabout, forms an aspirator which draws fluid from outer compartment 70 into the water stream passing from inner compartment 74 into downcomer nozzles 77, as indicated by arrows 94. The velocity of water discharged from the lower end of cylinder 76 is high (20 ft./sec. or more) and it acts as an aspirator or jet pump to withdraw fluid from the outer compartment. Maintenance of a limited but continuous flow of fluid in outer compartment 70 ensures a uniform temperature distribution over the entire inner surface of drum 10 without undue thermal stressing thereof. it might also be noted that a small pressure drop occurs in that steam-water mixture passing through holes into outer compartment 70 and this pressure drop flashes that mixture into superheated steam which has better thermal insulative properties than either water or saturated steam.

While the specific sizes, numbers and locations of holes 90 in outer partition 54 would depend on the specific design condition in a drum, the total free flow area of all such holes 90 should be considerably less than that of the openings through steam separators 86 which provide communication between the intermeidate compartment 72 and inner compartment 74 so as to both limit the flow of steam-water contacting the inner wall surface of drum l0 and to maintain sufficient flow velocities of the steam-water mixture through separators 86 to effect the required separation of steam and water. While either holes 90 or the communication between the outer compartment 70 and inner compartment 74 afforded by cylinder 76 can alone provide substantial pressure equalization across outer partition 54, both are necessary to provide both pressure relief and avoidance of the formation of a liquid-gas interface in outer compartment 70.

As best seen in FIG. 4, openings 96, similar to holes 90, are provided in outer partition 54 very near its longitudinal extremes and/or in closure plates 78, if both outer and inner partitions 54 and 56 respectively, are extended and secured in fluid tight relation to the end walls of drum l0. Openings 96 serve to extend outer compartment 70 into the space between closure plates 78 and the end of drum 10. Thus it is seen that the intermediate compartment 72 formed by outer and inner partitions 54 and 56 respectively, serves to greatly limit the quantity of furnace wall tube water-steam flow which is allowed to contact the inner wall surface of drum 10, while still achieving the requisite steam and water separation within the drum. While outer partition means 54 might be formed to extend outer compartment 70 about all or most of the drum end area, this construction would be difflcult to work with and is generally not necessary because the hemispherical drum ends may be thinner than the cylindrical drum walls and thus less subject to the problems occasioned by large thermal gradients. Further, the drum ends are not subjected to the high local peak stresses which occur at and near the many vessel penetrations in the cylindrical drum wall.

In addition to the protection from large thermal gradients which the fluid space of outer compartment 70 affords the wall of drum 10, further protection may also be derived by making at least the outer partition 54 of a material having a coefficient of expansion initially the same as that of drum but having a thermal conductivity substantially less than that of the drum. Typically, drum 10 is fabricated of carbon, low manganese steel and an appropriate material for baffle plates having the above specified properties would be the 431 series of ferritic stainless steel. Their coefficients of thermal expansion are substantially the same, but the thermal conductivity of the 431 series of ferritic stainless steel is approximately one-half that of the steel used for the drum walls. An outer partition formed of this stainless steel will accommodate changes in drum size due to temperature changes but will retard temperature transmission between intermediate compartment 72 and the wall of drum 10.

While the invention has been described in an embodiment employing a steam generator of the controlled circulation type, it should be understood that the general structure and principals of the invention are generally applicable to a natural circulation system.

While this preferred embodiment of the invention has been shown and described, it will be understood that it is merely illustrative and that changes may be made without departing from the scope of the invention as claimed.

What is claimed is:

1. In combination with a steam generator, a generally horizontal steam and water drum having its interior divided by outer and inner partition means into an outer substantially annular compartment generally coaxial with and extending longitudinally of and coextensive with the drum with the inner wall surface of said drum forming the outer wall thereof and said outer partition means forming the inner wall thereof, an intermediate compartment extending longitudinally of and substantially coextensive with the drum and spaced from the inner wall surface thereof with said outer partition means forming the outer wall thereof and said inner partition means forming the inner wall thereof, and an inner compartment, said inner partition means having openings extending therethrough, said outer and inner partition means being spaced from and sealed in fluid tight relationshipto one'another to form said intermenication means below said level of water in said inner I compartment, a steam generating circuit having its inlet in fluid communication with said inner compartment at a point below the water level therein and its outlet being in said intermediate compartment and means effective to circulate water from said inner compartment to said intermediate compartment via said steam generating circuit.

2. The apparatus of claim 1 wherein said outer and inner partition means are formed of thin section material and said outer partition means have holes therethrough located to provide limited fluid communication directly between said intermediate compartment and said outer compartment, the total free flow area of all said holes being substantially less than that of the openings in said inner partition means which provide communication between said intermediate compartment and said inner compartment.

3. The apparatus of claim 2 wherein said means for fluid communication between said outer and inner compartments include aspirating means near the bottom of said drum for removing fluid from the outer compartment by suction established by water flowing through said aspirating means from said inner compartment into the inlet of said steam generating circuit, whereby fluid is continuously removed from said outer compartment to prevent the formation of a liquid level therein.

4. The apparatus of claim 3 wherein the inlet to said steam generating circuit is a downcomer nozzle and said aspirating means include a fluid conduit with a first end communicating with said inner compartment and the other end extending into said downcomer nozzle, that portion of said conduit extending into said nozzle being spaced from the inner surface of said nozzle to create a fluid passageway therebetween in communication with said outer compartment.

5. The apparatus of claim 3 wherein said holes in said outer partition means providing limited fluid communication directly between said intermediate compartment and said outer compartment are located along said outer partition means near the top thereof, whereby fluid entering said outer compartment from said inner compartment is caused to pass through the entirety of said outer compartment from top to bottom.

6. The apparatus of claim 5 wherein at least said outer partition means consist of a material having a coefficient of thermal expansion which is substantially the same as that of the material forming the wall of said drum and a thermal conductivity which is substantially less than that of said material forming said drum wall. k i 

1. In combination with a steam generator, a generally horizontal steam and water drum having its interior divided by outer and inner partition means into an outer substantially annular compartment generally coaxial with and extending longitudinally of and coextensive with the drum with the inner wall surface of said drum forming the outer wall thereof and said outer partition means forming the inner wall thereof, an intermediate compartment extending longitudinally of and substantially coextensive with the drum and spaced from the inner wall surface thereof with said outer partition means forming the outer wall thereof and said inner partition means forming the inner wall thereof, and an inner compartment, said inner partition means having openings extending therethrough, said outer and inner partition means being spaced from and sealed in fluid tight relationship to one another to form said intermediate compartment with communication between said intermediate compartment sand said inner compartment established through said openings in said inner partition means, said inner compartment being at least partially filled with water, said outer compartment extending on each side of the drum below the level of the water in said inner compartment and being fluidly isolated from said inner compartment except for communication means below said level of water in said inner compartment, a steam generating circuit having its inlet in fluid communication with said inner comparTment at a point below the water level therein and its outlet being in said intermediate compartment and means effective to circulate water from said inner compartment to said intermediate compartment via said steam generating circuit.
 2. The apparatus of claim 1 wherein said outer and inner partition means are formed of thin section material and said outer partition means have holes therethrough located to provide limited fluid communication directly between said intermediate compartment and said outer compartment, the total free flow area of all said holes being substantially less than that of the openings in said inner partition means which provide communication between said intermediate compartment and said inner compartment.
 3. The apparatus of claim 2 wherein said means for fluid communication between said outer and inner compartments include aspirating means near the bottom of said drum for removing fluid from the outer compartment by suction established by water flowing through said aspirating means from said inner compartment into the inlet of said steam generating circuit, whereby fluid is continuously removed from said outer compartment to prevent the formation of a liquid level therein.
 4. The apparatus of claim 3 wherein the inlet to said steam generating circuit is a downcomer nozzle and said aspirating means include a fluid conduit with a first end communicating with said inner compartment and the other end extending into said downcomer nozzle, that portion of said conduit extending into said nozzle being spaced from the inner surface of said nozzle to create a fluid passageway therebetween in communication with said outer compartment.
 5. The apparatus of claim 3 wherein said holes in said outer partition means providing limited fluid communication directly between said intermediate compartment and said outer compartment are located along said outer partition means near the top thereof, whereby fluid entering said outer compartment from said inner compartment is caused to pass through the entirety of said outer compartment from top to bottom.
 6. The apparatus of claim 5 wherein at least said outer partition means consist of a material having a coefficient of thermal expansion which is substantially the same as that of the material forming the wall of said drum and a thermal conductivity which is substantially less than that of said material forming said drum wall. 